Well, you learn something every day. I suppose if you don't manage to learn as you go, then you probably stuff things up every day!!!
About 12 months ago I came across some RCF L18/551 eighteen inch subwoofer baskets. I reconed them using aftermarket parts, the process went OK but I was kind of bemused by the untreated cloth surrounds.
I was busy, and had no immediate plans so the drivers were shelved for a rainy day.
The Thielie and Small parameters were "OK" but not exact as OEM. I measured these using the addded mass method, an important fact for later. As an aside, they would do well enough as subs, but their Fs was a bit higher than I would have hoped.
Fast forward to last week, I loaded one into a 220 litre box tuned to 34Hz, and measured the impedance.
Bugger me if the resonance showed one massive peak at pretty much the driver Fs, and a tiny lump out at 80Hz. Huh?
I swapped drivers...